The present invention relates generally to Portland cement mixtures, and more particularly to Portland cement mixtures characterized by the addition of finely divided limestone.
Portland cement concrete has found increasing use in the construction industry since it was first introduced in the early 19th century. Since that time, there has been very little change in its composition. However, the modem economy has created new requirements for Portland cement manufacture and use, and those requirements have not been adequately met by advances in Portland cement compositions.
For example, environmental concerns have focused on the carbon dioxide emissions resulting from Portland cement manufacture. Carbon dioxide emissions are thought to contribute to the so-called xe2x80x9cgreenhouse effect,xe2x80x9d which is widely believed to have contributed to global warming. Portland cement manufacture has been a contributor to the emission of carbon dioxide into the atmosphere. One approach to this problem has been the increasing use of pozzolans and blast furnace slags as additions to Portland cement in order to decrease carbon dioxide emissions during its manufacture. Examples of such pozzolans include fly ash and silica fume. The introduction of pozzolans and slags into cementitious mixtures, including Portland cement, has become a well-accepted practice.
A disadvantage of introducing pozzolans or blast furnace slags into Portland cement mixtures is that it adversely affects two key measures of performance commonly used to assess cementitious mixtures. Specifically, pozzolans and slags are known to lengthen the initial setting time and decrease the rate of compressive strength gain of the resulting cementitious composition. This set and strength retardation caused by pozzolan and slag addition slows the rate of construction of the project for which the resulting cementitious composition is being used.
The need for both rapid hardening and early strength gain of Portland cement concrete is dictated by the cost of time in modem construction. The use of pozzolans or slags in such mixtures, although beneficial to the environment, has a direct and calculable cost due to the additional time needed to achieve initial set and to the additional measures that must be taken to ensure the requisite early strength of the construction project to which it is applied.
Certain accelerators have been developed to offset the decrease in rate of set and strength gain occasioned by the addition of pozzolans and slags. These include calcium chloride, calcium nitrite and calcium nitrate. However, calcium chloride creates the potential for corrosion of any steel reinforcement that is to be placed within the concrete, and calcium nitrite and calcium nitrate are relatively expensive and less efficient. The disadvantages associated with these known accelerators make them unsuitable for many applications.
What is needed is a Portland cement composition that includes an accelerator to increase the rate of initial set and the rate of strength gain without the disadvantages of the prior art accelerators. Such an accelerator should be particularly effective in the presence of pozzolans, blast furnace slags and the like.
Accordingly, an object of the present invention is to provide a Portland cement composition with a relatively low-cost accelerator for increasing the rate of initial set and rate of strength gain of the composition.
Another object of the invention is to provide a Portland cement composition with an accelerator that does not cause degradation of steel reinforcing placed in the composition.
Another object of the invention is to provide a Portland cement composition with an accelerator that offsets the slowing in initial set time and rate of strength gain of the composition caused by the addition of pozzolans and blast furnace slags.
In satisfaction of these and other objects, there is provided an improved cementitious composition comprising Portland cement and finely ground or divided limestone. This limestone, which is carbonate material composed chiefly of calcium carbonate, may be added to the Portland cement with or without pozzolans or blast furnace slag. In one embodiment, one or more pozzolans or slags are added to the Portland cement and limestone mixture. Whether or not pozzolans or blast furnace slag forms a part of the mixture, the finely divided limestone acts as an accelerator to increase the rate of set and the rate of strength gain of the resulting composition. In one aspect of the invention, the Portland cement is of fineness commonly associated with commercial Portland cement. Typically, these Portland cements have a Blaine fineness of at least about 3,200 sq. cm. per gram (cm2/g), and more typically are in the range of Blaine fineness from about 3,500 cm2/g to about 5,500 cm2/g.
In another aspect of the invention, when pozzolans or blast furnace slags are added to the composition, one or more pozzolans or slags are selected. In one embodiment, there is added class C fly ash, which is the particulate effluent from the burning of sub-bituminous coal. In another embodiment, there is added ground, granulated blast furnace slag. In yet another embodiment, pozzolans and slags are added in combination. Although other pozzolans or slags may be chosen in addition to, or as a substitute for, the foregoing, it has been discovered that the effect of the finely divided limestone on rate of set and rate of strength gain is particularly pronounced in the presence of these pozzolans or blast furnace slags. Moreover, the effect is even more pronounced as these pozzolans or blast furnace slags are added in increased amounts to dilute the Portland cement mixture.
The invention achieves the above and other objects through the use of finely divided limestone in proportions to Portland cement and pozzolans or blast furnace slags not heretofore known or appreciated. Limestone has previously been used in masonry cementing compositions and the like to increase grinding efficiency, achieve color compatible extensions in volume, lower cement strength potential, lower heat of hydration, or increase workability. However, the present invention is characterized by the use of limestone as an accelerator to increase the rate of initial set and the rate of strength gain, particularly in the presence of pozzolans or blast furnace slags.
As previously pointed out, preferably the inventive composition comprises Portland cement having a fineness of at least about 3,200 sq. cm/gm (Blaine). In practice, Portland cements more coarse than about 3,200 Blaine are not expected to exhibit the degree of reactivity that is suitable for ordinary construction.
The invention employs finely divided limestone, which in practice may exist as part of bulk limestone with some particles lacking the requisite fineness. Those or ordinary skill in the art will recognize that both the calcium carbonate content of the limestone and the proportion of the bulk limestone having the requisite fineness can be expected to affect the gross amount of limestone necessary to be mixed for achieving the desired result. For example, a sample of graded limestone sand with 50% of its particles larger than the standard #100 U.S. sieve will require about double the amount of total limestone content in a given cementitious composition in order to achieve equal reactive presence of limestone with no particles larger than #100 sieve, where the requisite level of fineness is about #100 sieve. Similarly, a limestone with substantial alternative mineral content (e.g., magnesium) will require proportionally increased addition.
Preferably, finely divided limestone is added to mortars containing Portland cement, sand and water. Pozzolans, blast furnace slag, or both are also added. The Portland cement, pozzolans, and slags form a first cementitious mixture, wherein the weight ratio of minus #100 sieve (150 micron) limestone to the resulting mortar mixture (cement, limestone, pozzolan and/or slag, sand and water) ranges from about 30:2200 to 120:2200 or 1.4% to about 5.6%. Any of the pozzolans can be used, including class C fly ash, class F fly ash, or silica fume. These pozzolans and slags may be used individually, or added to the composition in various combinations with each other.
The limestone is preferably naturally occurring limestone, commonly about 50% to 95% calcium carbonate by weight. There are many common commercial geographical sources of limestone including those from Atlanta, Florida, Oklahoma and Detroit. The efficacy of the limestone addition increases with decreasing particle size and increasing calcium carbonate content. The limestone is finely divided, preferably having fineness at least minus # 100 U.S. standard sieve (150 micron). As shown in the examples below, the characteristics of increased rate of set and rate of strength gain are also exhibited by cementitious compositions of Portland cement with finely divided limestone even in the absence of pozzolans or blast furnace slag. Although the effect on rate of set and rate of strength gain is most pronounced in the presence of pozzolans or slags, and more particularly in dilute compositions having higher concentrations of pozzolans or slags, these effects are also exhibited in the absence of pozzolans or slags.